Familial adenomatous polyposis (FAP) is an inherited condition in which numerous polyps form mainly in the epithelium of the large intestine. While these polyps start out benign, malignant transformation into colon cancer occurs when the polyps are not treated. FAP can occur due to mutations in the Adenomatous Polyposis Coli (Apc) gene. The product of the Apc gene is a 300 kDa cytoplasmic protein associated with the adherence junction protein catenin. Humans with mutations in the Apc gene get hundreds to thousands of adenomas in the colon and typically develop colorectal cancer.
Experimental analysis of human colon cancer aims to develop an accurate model for studying tumor initiation, progression, and potential treatments. Much of modern cancer research is predicated on the establishment of animal tumor models, in particular rodent models. Murine (mouse) models are widely used. Traditionally, the rat has been favored for physiological studies while the mouse has been preferred for genetics.
One of the most widely distributed mouse models for studying human intestinal cancer is the ApcMin (Min) mouse, with a two month growth period for macroscopically visible, countable tumors. The ApcMin mouse was obtained through the use of chemical mutagenesis using N-ethyl-N-nitrosourea (ENU), and was described by Moser et al., 1993, Proc. Natl. Acad. Sci. USA 90: 8977-8981. The ApcMin mutant mice carry a single nonsense mutation in the adenomatosis polyposis coli (Apc) gene that results in a multiple intestinal neoplasia (Min) phenotype in affected mice—hence the name ApcMin.
While the Apc gene is typically altered in humans similarly affected by colorectal tumors, there are important differences between the manifestations of the disease in humans and mice. The Min mouse and related mouse models including Apc1638N/+ and ApcΔ716 are not without their disadvantages. One of the main criticisms of the models is that the tumors arise primarily in the small intestine, in contrast to the colonic localization in human FAP patients. Mice are rarely used as models in major animal drug trials owing to their small size, short life span, and low amounts of recoverable plasma. Additionally, Min mice do not develop metastases to distant sites. Finally, mouse chromosomes are acrocentric, yet the analysis of mechanisms of loss of heterozygosity requires data on both arms of a metacentric chromosome. Although the Rb(7.18)9Lub metacentric fusion line of mice provides a means to confirm somatic recombination in the Min mouse (Haigis and Dove, 2003, Nat. Genet. 33: 33-39), this translocation does not reflect the natural state, and the mice have a low colonic tumor incidence and multiplicity (21% incidence, <0.4 tumors per mouse). These differences are sufficiently significant to have prompted many researchers to use an alternative model, namely rats fed the colon carcinogen azoxymethane (AOM), to more closely mimic the etiology of the disease in humans.
The laboratory rat presents several advantages over the mouse: an increased size and longevity, an increased tolerance of tumor burden, and an overwhelming use for large-scale chemoprevention studies. Most important, the metacentric structure of rat chromosome 18 provides new ways to study the mechanisms of Apc loss of heterozygosity (LOH) that cannot be studied endogenously in the mouse and that cannot be manipulated in the human. The importance of the rat centers around a debate on the role of genomic instability in colon cancer, and whether genomic instability is necessary for tumor initiation, or genomic instability is acquired afterwards and either stimulates neoplastic progression or simply accumulates as the neoplasm advances. The rat could thus provide a metacentric model on which to test these hypotheses. However, at present, no embryonic stem cell lines exist for the rat, precluding knockout technology via traditional homologous recombination.
It would be advantageous to create a new animal model system for mimicking disease progression, prevention, and treatment for evaluation of human therapeutics in human intestinal cancer, including colorectal cancer. Such model system could be used for screening compounds for carcinogenic activity, as well as screening compound for carcinogenesis inhibitory activity. The present invention addresses these and other related needs.